macOS: Print from web view

[teusbenschop]

Printing the current page does not yet work in the new macOS app written in Swift.

• [x] Read how to print from WKWebView: https://stackoverflow.com/questions/33319295/how-does-one-print-all-wkwebview-on-and-offscreen-content-osx-and-ios
• [x] Read https://stackoverflow.com/questions/46777468/swift-mac-os-blank-page-printed-when-i-try-to-print-webview-wkwebview
• [x] Catch Command-P.
• [x] Catch the "Print" action to the menu.
• [x] Send notification about printing to the view controller.
• [x] Implement print.
• [x] Release.